In the first video standards, the video is assumed to be rectangular and to be described in terms of three separate channels: one luminance channel and two chrominance channels. The luminance signal is carrying the varying black and white information on a given amount N of bits (eight bits for instance). Each of the two chrominance channels contains a digital signal equal to a value comprised in the range defined by a chrominance representation on a given amount M of bits (eight bits for instance: with such a representation on eight bits, the values of the signal may vary between 0 and 255). In case of an only black and white signal, each of the chrominance channels contains a flat signal equal to a constant value 2M-1. However, with the above-cited standards, the syntaxes describing the signals to be transmitted always assume that the video is “coloured”. Although there are many video contents that are in black and white, said syntaxes force the transmission of chrominance descriptive elements which are not necessary.
The consequence of this lack of flexibility is a waste of bits, leading to a loss of coding efficiency illustrated for example in the case of the standards MPEG-4 and H.26L: (a) standard MPEG-4: as defined in page 134 of the MPEG-4 document w3056, also referenced “Information Technology—Coding of audio-visual objects                Part 2: Visual”, ISO/IEC JTC1/SC29/WG11, Maui, USA, December 1999, a variable length code, called dct_dc_size_chrominance, is used as a descriptive element encoding the size of the differential value that should be read from the transmitted or stored bitstream in order to update in the blocks of all intra macroblocks the last decoded chrominance DC coefficient and therefore obtain its current value. This differential value, equal to 0 when the chrominance signal is a constant value, is then encoded as “there is no differential value to be read” (i.e. dct_dc_size_chrominance=0), which results, as shown in the first line of Table B-14, p. 343, of said document, in a codeword of 2 bits per macroblock and per chrominance channel in I-pictures. For a CIF I-picture of size 352×288 pixels comprising 396 macroblocks, this leads consequently to a waste of bits of at least 396×2 channels×2 bits=1584 bits (at least, because, in fact, other syntatic elements can further degrade the coding efficiency and must also be taken into account in this evaluation).        
(b) standard H.26L: as defined in page 16 of the H.26L document Q15-K-59 “H.26L Test Model Long Term Number 5 (TML-5)-Draft 0”, ITU-Telecommunications Standardization Sector, 11th Meeting, Portland, Oreg., USA, Aug. 22-25, 2000, a so-called Coded Block Pattern (CBP) is used to indicate for any given 16×16 pixels macroblock which 8×8 blocks (for the luminance and for the chrominance) contain transform coefficients, i.e. in fact to indicate two kinds of information: which 8×8 luminance blocks were encoded in the bitstream (on 4 bits), and whether or not chrominance coefficients were encoded (3 possibilities, coded on 2 bits). This CBP element is then further encoded by variable length codes: the luminance and chrominance block patterns are jointly entropy-coded using a VLC table optimized for coloured sequences. However, this table is not optimized for black and white sequences: only 16 CBP values out of 48 are actually used, and the shortest VLC words are reserved for CBP values that are never encountered. It is difficult to precisely quantify this waste of bits, but it obviously exists.